Manuscript received March 3 I;
1988. This work was supported by the EEC under Contract ST2J-0171-2-DK. X. Pan is with the Electromagnetics Institute;
Technical University of Denmark;
DK-2800;
Lyngby;
Denmark. H. Olesen and B. Tromborg are with the Telecommunication Research Laboratory;
DK-2970;
H$rsholm;
Denmark. IEEE Log Number 8823596;